It's long ago that Serena and Venus were both at the top of the rankings. In fact they both really had a slump because of long injury-breaks. Now Serena is # 1 for more than a year and Venus is # 2 and leader of the Champions-Race. They are # 1 in the doubles rankings as well as in the doubles race.

I would say they don't play enough to rule the entire tour. Additionally they are not quite strong enough on clay to rule that part of the season. But outside of clay they are the favourites to take all the other really big titles. So while I wouldn't quite say they rule the entire tour they are proportioned a very big and important section of that tour with which they reign supreme.